1. Field of the Invention
The present invention relates in general to a hybrid drive system for a motor vehicle, and more particularly to techniques for smooth brake application by a motor/generator or an engine.
2. Discussion of the Related Art
JP-A-7-67208 discloses an example of a known hybrid drive system of a motor vehicle, which is equipped with a drive power source including an engine operated by combustion of a fuel, and an electric motor operated with an electric energy, and an automatic transmission which is disposed between the drive power source and drive wheels of the vehicle and whose speed ratio is variable. This type of hybrid drive system has a regenerative braking mode in which the motor/generator is operated by a kinetic energy of the running motor vehicle, to apply to the vehicle a regenerative braking force corresponding to an amount of electric power generated by the motor/generator. The braking force to be applied to the vehicle can be smoothly changed by controlling the amount of electric power or the amount of regenerative braking torque generated by the motor/generator.
When the automatic transmission is shifted from one position to another in the regenerative braking mode, a braking force generated by a resistance to rotation of the engine operatively connected to the automatic transmission tends to vary to a considerable extent, causing a change in the overall braking force applied to the vehicle, unexpectedly to the operator of the vehicle. When the vehicle is running with the motor/generator being connected to the automatic transmission, the regenerative braking force itself of the motor/generator varies upon a shifting action of the automatic transmission.
In some running condition of the vehicle, the regenerative braking force generated by the motor/generator is not sufficient for braking the vehicle as desired. In the event of some electrical abnormality of the hybrid drive system, the motor/generator may not be used to apply a regenerative braking to the vehicle. In such cases, the regenerative braking cannot be applied to the vehicle as desired by the vehicle operator.
Japanese Patent Application No. 7-294148 (not published before the present invention was made) discloses a hybrid drive system having a plurality of operation modes which include the regenerative braking mode described above, and an engine braking mode in which the engine is operated by a kinetic energy of the running vehicle, to apply an engine brake to the vehicle due to a resistance to rotation of the engine. The regenerative braking and the engine braking are collectively referred to as "drive-source braking", and the sum of the regenerative braking force and the engine braking force is referred to as "drive-source braking force".
In the above type of hybrid drive system, the regenerative braking mode and the engine braking mode are selectively established depending upon the amount of electric energy stored in an electric energy storage device, in order to prevent excessive charging of the storage device and resulting deterioration of the charging and discharging efficiencies of the storage device. Described in detail, the regenerative braking mode is established to charge the electric energy storage device when the amount of electric energy stored is smaller than a predetermined threshold level below which the storage device is not excessively charged by the regenerative braking. When the amount of electric energy stored in the storage device is larger than the threshold level, the engine braking mode is established to prevent excessive charging of the storage device.
In the hybrid drive system wherein the generative braking mode and the engine braking mode are selectively established on the basis of the amount of electric energy stored in the electric energy storage device, the operation mode of the hybrid drive system may be changed from one of the regenerative and engine braking modes to the other if the amount of the stored electric energy is considerably changed in the above-indicated one braking mode. Namely, the braking source (drive power source) used for braking the vehicle may be changed during the drive-source braking operation. In this event, clutches or other members provided for switching the braking source are operated during the braking operation, whereby the drive-source braking force tends to vary, unexpectedly to the vehicle operator. A similar problem is encountered where the generative and engine braking modes are selected on the basis of a required braking force or other vehicle running condition other than the amount of electric energy stored in the electric energy storage device.